simplifications in the ei_solve_impl system, factor out some boilerplate code
diff --git a/doc/snippets/HouseholderQR_solve.cpp b/doc/snippets/HouseholderQR_solve.cpp
index 429bd81..8cce6ce 100644
--- a/doc/snippets/HouseholderQR_solve.cpp
+++ b/doc/snippets/HouseholderQR_solve.cpp
@@ -4,6 +4,6 @@
cout << "Here is the matrix m:" << endl << m << endl;
cout << "Here is the matrix y:" << endl << y << endl;
Matrix3f x;
-m.householderQr().solve(y, &x);
+x = m.householderQr().solve(y);
assert(y.isApprox(m*x));
cout << "Here is a solution x to the equation mx=y:" << endl << x << endl;